How To Size A Whole House Water Filter System (Updated 2023)

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How To Size A Whole House Water Filter System

Water filtration system sizing is trickier than it may seem at first. After all, the wrong size filter can cause a drop in your water flow rate and ineffective contaminant removal. While getting one that is too big will cost you more money than necessary. So, you may be wondering how to size a whole house water filter system.

A correctly sized whole house water filter system is based on the required flow rate. A home’s flow rate is typically between 7 – 10 GPM but depends on water usage, the size of the home, and plumping configuration.

It seems daunting at first, but after reading this guide you will be able to select the proper size whole home filtration system for your home with confidence.

Why Proper Filtration System Size Matters

A whole house water filter works by water flowing through the filtration media at the point of entry of the home to remove unwanted contaminants.

The correct water filter size matters because it determines how much water can be filtered without constraining the water flow. If the filter is too small, it will significantly reduce water pressure and cause issues in every fixture and appliance. The last thing you want is to purchase a whole house water filter to solve a problem, only to create another.

On the other hand, if the filter is too large, it will be an unnecessary expense in terms of both upfront cost and ongoing maintenance. Larger filters will have greater filter replacement costs. They will also be prone to using more water if the system requires backwash cycles. It is important to choose a filter size that is appropriate for your needs to ensure that your water is properly filtered.

Sizing Measurement Units 

When searching for a whole home water filter, you’ll frequently find that the flow rate measurement is taken in GPM (gallons per minute).

In a nutshell, the flow rate of the water must be set at a specific level for the filtration media to collect all potential contaminants. If the capacity of your water filtration system is insufficient for the amount of water you go through, then the system will not be able to efficiently clean the water you use.

You may also see filtration options sized according to the number of bathrooms in a home. For example, a filter may be suitable for a home with 1 – 3 bathrooms, or 4-6 bathrooms. Keep track of these points as it is a useful estimate that you can use in sizing calculations.

How To Size A Whole House Water Filter

Understanding the nuances of filtration system sizing is critical to selecting the optimal model for your home. A lot of people get confused with all the variables and industry jargon when trying to decide on the right model. The reality is there are only a few factors you need to know to calculate the properly sized water filter:

  • Water flow rate
  • The water filter’s port size
  • Your home’s plumbing size

If you’re wondering what size whole house water filter do you need? We will walk through all of these calculations step-by-step to show you how to size your water filter system.

How To Size A Whole House Water Filter System_Water Flow Rate

Calculating Your Water Flow Rate

The amount of water used in your bathroom, hot water heater, dishwasher, and other appliances is directly proportional to the flow rate, measured in gallons per minute (GPM). When figuring out what whole home filter system you need, understanding the water demand ratings of your appliances and the water filter is a must. 

Your water flow rate requirements for a typical residential home range anywhere from 6 to 40 gallons per minute (GPM), depending on the size of your home and family. It is generally undesirable to have a whole-house filtration system with a flow rate of less than 6 GPM to maintain continuous water pressure during peak usage.

Calculating Flow Rate By Number Of Appliances

An easy way to calculate a home’s flow rate is by adding up the water consumption per fixture and appliance. A typical family requires a staggering 300 gallons of water per day, at a flow rate of 6 -12 gallons per minute.

For example, a typical shower uses 2 gallons of water per minute. If your home has 3 bathrooms with all three showers turned on simultaneously, you’ll be using at least 6 gallons per minute (GPM). However, toilets are the biggest source of water use in a home, accounting for approximately 30% of total consumption.

Here are the estimated water consumption rates per appliance:

Appliance Water Consumption
Shower2 GPM – 4 GPM
Toilet2 GPF – 5 GPF
Washing Machine3 GPM – 6 GPM
Dishwasher2 GPM – 4 GPM
Kitchen Sink2 GPM – 3 GPM
Bathtub3 GPM – 7 GPM

Keep in mind that these are average flow rates per water appliance and that many appliances will provide precise usage information for you to use.

Calculating Flow Rate For Individual Taps And Fixtures

The more precise way to calculate the water flow rate is by examining each individual fixture and appliance. You can measure the flow rate of a tap or fixture by tracking the time it takes to fill a container. With this method, you will need to know the flow rate of each appliance and the total time that they are in use.

Using the bath faucet as an example. To do this you can get a 1-gallon container and place it under the faucet. Turn the water faucet all the way on and begin timing with a stopwatch or mobile phone as the container fills. Once the container is filled, turn off the faucet completely and stop the time clock.

The faucet GPM is calculated as follows: 60 ÷ time to fill x gallons filled

You can use this formula to calculate the flow rate for any number of home fixtures. Then you have the precise flow rate for your home’s water fixtures. Make sure to convert the flow rates to gallons per minute if given in a different unit, such as liters per minute.

Calculating By Number Of People And Bathrooms

Another way to calculate the water flow rate is to multiply the number of people in the home by the number of bathrooms. This method is a rough approximation, but not as precise as measuring the water use per appliance. Simply using the number of bathrooms is good, but it doesn’t tell the whole story. We also need to know the water usage from the number of people in the home.

For example, an average family of 4 in a home with 3 full bathrooms will require a flow rate of approximately 10 GPM. And here is an important tip, always round up! If you are looking at a range between 7 – 10 GPM for your home, get a system that will accommodate at least 10 GPM.

Here is a matrix comparing bathrooms and number of people to estimate your water flow rate:

# Of People1 Bath2 Bath3 Bath4 Bath5 Bath6 Bath7 Bath
49 GPM10 GPM11 GPM12 GPM12 GPM13 GPM14 GPM
510 GPM12 GPM13 GPM13 GPM14 GPM15 GPM16 GPM
611 GPM13 GPM14 GPM15 GPM16 GPM17 GPM18 GPM
712 GPM14 GPM15 GPM17 GPM18 GPM19 GPM20+ GPM

You can use these estimates as a starting point and adjust them based on your circumstances. Keep in mind, this is an approximation. Each home uses a different amount of water per person. It’s also a good idea to monitor your water use over time to get a more accurate picture of your water flow rate.

Sizing Based On Plumbing And Port Size

Filter Port Size

The standard for whole-house water filters is typically a one-inch port size. Under most circumstances, you will not want to go smaller than a 3/4 port size to avoid any bottlenecks. If you use a 1-inch port size or better, pressure drops should be negligible. That said, be sure to consult the flow rate options listed above, and compare that to the port sizes available for the particular filtration system model.

Proper Pipe Size

You should also examine the size of your home’s plumbing pipe size. A whole house water filter is a point-of-entry system, so they are installed at the main entry line of your home. You will need to know this to understand what filters are compatible, what plumbing modifications may need to be made, and what flow rate you can expect.

Since the amount of water flowing through the line pipe is a good indicator of the filter’s capacity, measuring the diameter of the pipe is an excellent first step. One-inch line pipes, for instance, typically have a flow rate between 9 and 27 gallons per minute.

Pipe SizeMinimum Flow Rate (GPM)Maximum Flow Rate (GPM)
½ Inch Pipe3 GPM6 GPM
¾ Inch Pipe5 GPM15 GPM
1 Inch Pipe9 GPM27 GPM
1 ¼ Inch Pipe15 GPM35 GPM
1 ½ Inch Pipe25 GPM50 GPM
1 ¾ Inch Pipe30 GPM70 GPM
2 Inch Pipe60 GPM110 GPM

What Is a Typical Home Water Flow Rate And Filter Size?

The water flow rate in a typical home is usually between 8 – 10 gallons per minute. However, this can vary based on the size of the home, the number of bathrooms, the age of the plumbing, and other factors.

The most common size whole house water filter on the market is 4.5″ by 20″ for a cartridge system. A whole house tank based filtration system will measure approximately 48″ tall by 9″ wide. These will generally be suitable for most homes, but you may need to increase the size as your home’s water usage increases.

Water Flow Rate VS Water Pressure

Some individuals interchange water pressure and flow rate. Despite their connection, the two are still distinct factors. If the water pressure is increased, the flow rate will also increase, although pipes of a given size have flow restrictions.

How Does Flow Rate Impact Filtration?

Assuming your whole-house filter can handle the increased flow rate, filtering time will decrease. Ultimately, the filter functions as an entrance through which your water goes. Thus if you boost the flow rate, more water will travel through the filter. This will always result in faster filtering if you don’t exceed the filter’s maximum flow rate.

Why Water Pressure Is Critical

Maintaining the proper water pressure in your pipes affects more than just how quickly the water drains. Incorrect water pressure can cause issues with your plumbing, including irreversible damage to piping or fixtures. This is true both for low-pressure and high-pressure situations.

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